Lighting the way: IIHS headlight ratings predict nighttime crash rates

Brumbelow, Matthew L.
Insurance Institute for Highway Safety
October 2021

Introduction: Vehicle headlights are the primary means of providing visibility illumination for drivers at night, when crash rates are several times higher than during the day. Based on research indicating a wide range of headlight performance in the passenger vehicle fleet and the absence of a comprehensive and objective consumer evaluation program, the Insurance Institute for Highway Safety (IIHS) began testing and rating headlight systems in 2015. The purpose of this study was to examine the relationship between headlight visibility, as quantified by IIHS, and real-world crash occurrence. Material and methods: Poisson regression was used to estimate the effects of the headlight rating and the underlying demerits on the rate of nighttime single-vehicle crashes per vehicle mile traveled, while controlling for differences in daytime crash rates and other factors.
Results: Vehicles with better headlight visibility have lower nighttime crash rates. Achieving 10 fewer visibility demerits, the equivalent of one overall rating band, was estimated to reduce the nighttime crash rate by 4.6% (95% confidence interval [CI]: 2.1%–7.0%). While statistical significance was limited by small sample sizes, good-rated headlights were estimated to reduce crash rates by 12 to 29% relative to those with poor ratings for the different types of single-vehicle crashes studied. Among different components of the IIHS rating, the assessments of low- and high-beam curve visibility were associated with the greatest crash rate reductions.
Conclusion: This study demonstrates that the IIHS evaluation program encourages headlight designs that reduce the risk of nighttime single-vehicle crashes.